1. Field of the Invention
The present invention relates to dry cell plates in which specified chemical compositions are used as active materials for forming electrodes, especially where electrodes of zinc or zinc-base alloys are employed and, more particularly, it relates to chemical preparatory coatings for zinc-containing metals using essentially aqueous solutions to form with zinc surfaces an intermediary layer having significant properties including very low resistivities, high adhesion, and coupling characteristics.
2. Description of the Prior Art
It has long been known that conversion coatings can be created on metals, especially zinc, steel, iron, aluminum and their alloys, by coating solutions of specific compositions have been used for this purpose. For example, the formation of coatings on metal often results from the chemical intereaction between acidic solutions and the metal surface, such as in an etching process. Admittedly, particular industrial significance has accrued over the years to these sundry chemical treating processes which include phosphating, chromating, oxalating, alkaline oxidation and the like.
With particular reference to zinc and its various alloy forms it has been customary heretofore in the manufacture of battery plates and, especially for those used in the production of zinc dry cells, that the zinc metal be pretreated to make it more receptive to conductive overlayers. As a matter of fact, zinc and its alloys are used rather extensively in the battery industry. It is known that zinc is electrochemically active, shows high hydrogen overvoltage, is low polluting and is still low priced; accordingly, it is suitable as an electrode material giving high energy density, high output, low self-discharge, low pollution and low price.
It has been the practice in the past to merely employ roughening and/or cleaning means over the metal surface in order to make it properly receptive to an overcoating of an electrical conducting material, this latter coating being often carbon or graphite-containing compositions. The commercially accepted procedure in the past has been to simply pit or indent the metal surface by exposing the metal to a mechanical roughening process such as sand blasting which has been the conventional technique employed in the preparation of stock materials for battery electrodes. Such stock materials are roughened by simply blasting them with the abrasive with attendent removal of surface metal and any oxides, along with residual materials. It is known that while sand blasting treatment produces a surface which provides a better adhesion surface for metals such as zinc than one that is untreated, the adhesion characteristics are generally only moderately good and therefore far from being completely satisfactory. Other disadvantages of the conventional practice of sand blasting is that it often allows abrasive particles to be imbedded in the metal surface, that it is costly, time consuming, noisy, inconvenient and that it could present a health hazard. The subject invention attempts to circumvent these problems and at the same time provide a straightforward procedure for treating metal surfaces used in battery plates in that, inter alia, it offers an improved method for bonding electrically conducting materials to metals. Seemingly, attempts in the past to form appropriate coatings have not been pursued for one reason or another. Moreover, if there have been attempts, they have often failed because such treatments did not adhere well or they interfered with the interplay of electrical phenomena associated with the working battery cell.
Thus, although surface roughening generally has been utilized, chemical pretreatment methods wherein solutions of various compositions are employed have generally not been widely resorted to in the art.
U.S. Pat. No. 2,860,039 to Marguller et al describes graining of zinc metals for lithographic plates by employing 1 to 5 weight percent mineral acids, 1 to 10 weight percent hydrogen peroxide and the remainder water.
U.S. Pat. No. 3,620,770 to Yoshikara et al teaches a chromate conversion coating for zinc for receiving paint, the coating solution being an aqueous acid solution containing hydrazine and ions of chromium and iron.
There are many electrically conductive coating compositions which can be used as conductive overlayers on zinc electrodes used in dry cells. Most of these consist essentially of a chemically stable, film forming organic polymer or binder blended with electrically conductive fillers such as finely dispersed graphite or carbon. A requirement of the fillers is that they be capable of conducting current without interfering with the basic electrochemical reactions taking place in the battery. Because of this, most finely dispersed metallic fillers as conducting materials cannot be used. The carbon-polymer conductive coatings are applied over the prepared zinc surfaces normally to a dry film thickness of about between 3 and 5 mils. The purpose of such a coating is to prevent chemical attack on the zinc surface by the battery electrolyte and to conduct current being generated within the cell. As a result, this requires that the conductive coating maintaining good adhesion to the zinc surface and to retain low electrical resistivity throughout the life of the battery.